I’m guessing, since Themes and styles can vary, but you might look for #postmetadata, or generate a test page and view the source and search for “Posted in Category” and see what section that shows up in. That will lead you to the right place in the CSS file.

In the Default theme (Kubrick):
For example index.php – line 17.<p class="postmetadata">Posted in <?php the_category(', ') ?>...etc.
In the same way you can find it in similar context in ALL the pages that use The Loop: archive.php, search.php, single.php.